Revert "Make EfficientLength public, as EfficientLengthIterable."

sdk/lib/_internal/compiler/js_lib/linked_hash_map.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Efficient JavaScript based implementation of a linked hash map used as a
 // backing map for constant maps and the [LinkedHashMap] patch
 
 part of _js_helper;
 
 const _USE_ES6_MAPS = const bool.fromEnvironment("dart2js.use.es6.maps");
@@ skipping 352 matching lines @@
   final hashMapCellKey;
   var hashMapCellValue;
 
   LinkedHashMapCell _next;
   LinkedHashMapCell _previous;
 
   LinkedHashMapCell(this.hashMapCellKey, this.hashMapCellValue);
 }
 
 class LinkedHashMapKeyIterable<E> extends Iterable<E>
-                                  implements EfficientLengthIterable<E> {
+                                  implements EfficientLength {
   final _map;
   LinkedHashMapKeyIterable(this._map);
 
   int get length => _map._length;
   bool get isEmpty => _map._length == 0;
 
   Iterator<E> get iterator {
     return new LinkedHashMapKeyIterator<E>(_map, _map._modifications);
   }
 
@@ skipping 32 matching lines @@
     } else if (_cell == null) {
       _current = null;
       return false;
     } else {
       _current = _cell.hashMapCellKey;
       _cell = _cell._next;
       return true;
     }
   }
 }